We all breath the same air and it can never be fresh again. A CO2 of 420 ppm is past the threshold of having increasing subtle effects of human brain function. This may as yet be more of a psychological effect of Unconscious CO2 detection by sensory chemical means. CO2 is a ubiquitous part of acid base balance in every cell of our body, and a key regulator of our rate of breathing. As it gets hotter we also make a stronger drive for fresh air, away from the stuffiness of our breathing out fellows. When the air gets stuffy so do our brains which operate on the edge of high metabolism, like a micro cpu hitting its heat sink temperature limit, we start to not function. Global human brain failure can and will be a real thing.